It is well known that the antenna or the preselector of a radio receiver must be properly tuned to achieve maximum sensitivity. The prior art shows a variety of circuits and techniques for tuning radio receiver antennas and preselectors.
For example, U.S. Pat. 4,837,852 (Takada) describes a circuit for implementing the conventional tracking approach to tuning an antenna wherein the same voltage that changes the frequency of the local oscillator changes the antenna tuning. A circuit diagram of a prior art tracking type of antenna tuning circuit is shown in FIG. 1 of this application.
U.S. Pat. 4,789,866 describes the use of varactor diodes to tune an antenna system to a desired frequency.
U.S. Pat. 4,381,566 describes a circuit for tuning a small sized high gain antenna by use of a distributed constant loading element combined with a tuning unit.
The problem of antenna tuning is particularly acute in small low power radio receivers such as those used in wristwatch pagers of the type shown in U.S. Pat. No. 4,713,808 (Gaskill). Such wristwatch radio paging devices use the wristband as an antenna, and differences in wrist size and variations in how the wristband is worn create significant tuning problems.
U.S Pat, No 4,817,196 (Macnak), U. S. Pat. No. 4,862,516 (Macnak) and co-pending application Ser. No. 07/279,952 (Gaskill) relate to tuning an antenna in a radio paging receiver which has a wristband antenna. The tuning techniques shown in these references rely on measuring the signal strength at the output of the receiver as the tuning is changed. The antenna is tuned so that the output signal is a maximum. Such circuits require a relatively long period of operation and sometimes they do not result in true optimum tuning.